The present invention relates to direct conversion, such as homodyne receivers, and in particular to circuitry for the correction of DC offset and second order intermodulation distortion in such receivers.
One of the major problems affecting direct conversion receivers is the presence of DC offsets in the base band in-phase and quadrature output lines. It is difficult to remove because most digital modulations have a DC component, which must be preserved. DC offsets are caused by local oscillator leakage back into the antenna port of the terminal and by circuit imbalances. AC coupling can be used to block DC offsets, but such coupling reduces the effectiveness of the demodulation because of the additional group delay applied to the signal. AC coupling can be used with low intermediate frequency (IF) conversion, where the down-converted channel has a normally small frequency offset so that no received components of the desired channel have a DC component, but these schemes have difficulty in obtaining adjacent channel selectivity.
Signal leakage from the radio frequency input to the local oscillator port, and circuit imbalance combined with non-linearity within the mixer and/or other components of the circuit (for example, amplifiers and filters), cause a second and more damaging form of DC offset. Both effects cause the radio frequency signal to mix with itself and generate a DC component that varies with the square of the amplitude of the input signal. This form of DC offset is a result of second order intermodulation distortion within the mixer and/or other components. Recovery of the demodulated base band signal by the direct conversion receiver can therefore be jammed by any large off-channel interfering signal, irrespective of its frequency. In TDMA systems this distortion causes the DC offset to pulsate with the transmission bursts of a strong nearby transmission. For transmissions employing non-constant envelope modulations, additional offset modulations will occur, these offset modulations can often have spectral components with bandwidths exceeding one channel bandwidth, and are therefore capable of causing interference to systems operating in the low IF mode. Unlike third order intermodulation effects, where the interfering signals must have a certain predefined frequency relationship before significant distortion is experienced second order intermodulation distortion can be found to be present in direct conversion receivers whenever large signals exist.
There currently exists a deed to effectively correct distortion introduced into direct conversion receivers by DC offset and second order intermodulation.
With this in mind, the present invention provides an distortion correction circuit for a direct conversion receiver, the direct conversion receiver comprising a local oscillator for generating a local oscillator signal, a mixer for multiplying a radio frequency signal, and a local oscillator signal together and supplying the resultant mixer output signal to a demodulated signal path, the demodulated signal path including a first low-pass filter for selecting a baseband signal at a first filter output terminal, wherein the offset correction circuit includes a squaring circuit for squaring the mixer output signal and supplying the resultant distortion estimate signal to a distortion estimate signal path, signal subtraction means for subtracting the distortion estimate signal at a subtraction point in the distortion estimate signal path, and adaptive processing means for equalising the transfer function of the distortion signal estimate path and the transfer function of the demodulated signal path between the mixer output and the subtraction point.
If the direct conversion receiver uses a quadrature demodulator then the distortion correcting circuit can be applied to each of the in-phase and quadrature arms.
In one embodiment of the invention, the signal subtraction point is between the mixer and the first low-pass filter.
In another embodiment, the distortion estimate signal path further comprises a second low-pass filter and the signal subtraction point is at the first filter output terminal.
Conveniently, the subtraction of the distortion estimate signal at the subtraction point generates an error signal, the adaptive processing means acting to equalise said transfer function by minimising the error signal.
The adaptive processing means may be an n-tap adaptive linear combiner.
The n-tap adaptive linear combiner preferably implements a least mean squared error algorithm.
The adaptive processing means may be implemented by digital signal processing.
The distortion correction circuit may further provide delay means for introducing a delay into the demodulated signal path, prior to the subtraction point
The adaptive linear combiner may include at least a first weight update coefficient for adjusting the weights of a plurality of said taps.
The adaptive linear combiner may include a DC tap to minimise a DC offset in the demodulated signal path.
The adaptive linear combiner may further include in this case, a second weight update coefficient for separately adjusting the weight of the DC tap.